When I have my truck in Park the rpms seem to jump a little bit. Up and Down In drive on the brake it's right on the park stays still etc... I was wondering what would cause this? a Vacuum leak?

I think my TB is leaking a tiny bit right on the engine.

 

then whats the question, change the gasket..

 

Wonder what would cause the idle to jump like that. Goes up and down about 100 rpm. This does not happen in any other vehicle I have sat in.

 

Varsis -

Am getting exactly the same thing. Just ran ~ 1/2 pint of sea foam cleaner thru the brake vac. assist line into/thru the TBI and top end of the engine. Let it sit for about 15 min's., restarted the engine. Smoked like hell for about 5 minutes (took a ride), then pretty much cleared up/clean now. In park/neutral, engine runs rough in the 300-50/600 RPM range; oscillates up & down. BUT - when in drive, stopped, running, etc., NO PROBLEM !

Have an 03 Dakota Sport QC 4x4, 4.7L V8 w/ ~ 69k miles on it. Been a peach so far.

Any other advice, thoughts, support - very appreciated ! Let me know what you find out......will also look for a vac. leak.

Did a lot of searching over this past weekend - possible sources (?): TPS, Battery, Vac. Leak, Coil (?), Fuel Filter, O2 sensors (doubt it), thermostat (?), Cat. Convertor (really reaching here)...... .

Let me know ..... FF

 

I think the question was would a vacuum leack cause this... Yes it could, but might not be the problem. Could be a bad sensor. Hard to tell... Let us know what you find.

 

I might try and remove the TB tomorrow and see if that will fix it clean it out etc... I did the seafoam trick to. Used a fair bit Blew a lot of smoke! I have a new converter, fixed manifold leak I believe. I think My next try is this. Most people I know the idle stays pretty steady, but I do hear a seeping noise on the rear end of the TB, I might try and see if I can fix. Hopefully I can.

According to the manual if you have a vacuum leak and you put water on the affected area the idle will change... so Maybe try this I did but nothing happened (Couldn't get the water in the right spot. ) Too cold to use the hose now too.

 

I think the idea behind the "water trick" is that the water will momentairily stop the leak- thus changing the RPM. If your RPMs are only fluctuating 100 or so, then it would be very hard to notice a change.
If you haven't yet, i would say deffinatly correct the TB vacuum leak, and while you have the TB off, clean it up really well- expecially where the butterfly valve meets the side of the TB. You might also clean the IAC while your at it...
Hope this helps....

 

def clean the tb and the iac when i did both of my they were filthy and covered in crude and after i did that the idle seemed to smooth out more and not bounce around i still get like 600-610 hoverage but thats it and most times its just pegged at 600. gonna do seafoam this weekend after i use the other half for my oil so hopefully that fully cures it. good luck.

 

Success ! Yee - haawwww !!!!

Well, folks......did the complete TBI removal & cleanup over last weekend. Removed the TB unit, took the IAC & the TPS off of the unit. Checked the TPS for the correct voltages (as is seen in several references), seemed to activate flawlessly. Cleaned this unit up - was coated with that fuel varnish/carbon stuff (note - ONLY used TBI cleaner, not some other cleaner. Don't want to cause seals/o-rings to deteriorate!). Then removed and also cleaned a more-encrusted IAC motor unit. Was really gummed up - I suspect that this was the source of the rough idle in neutral/park. Also, replaced the seal between the TB unit and the top of the intake manifold/fuel rail unit.

Reassembled everything, did the correct torque settings according to spec's., and VOILA ! The engine PURRS like a kitten next to a hot stove in a cold Vermont winter !

As time& funds allow, I also plan to run some sea foam into the oil (before an oil change - sea foam in, run 200-300 miles, then change it w/ filter) and into the gas tank to help clean up the injectors and the other parts of the top end of the engine. And lastly, I plan to - 1)remove the oil pan, clean up/out the pump/sending unit (sludge can accumulate there), 2) check the MAP sensor, 3) check the O2 sensors, and 4) to replace the plugs (~ 69k miles on originals) - assume that the Autolite 3923's are the tickets here. Should be great to go at this point ! In Vermont - predictions for cold, snowy winter here - got to get her in shape !

FWIW...........hope this helps others. FF

(PS - this forum is GREAT !!! Thanks to all the prior posters in this area !)

 

I hope to get around to doing this soon and Fixing my Manifold Leak (PAIN IN THE ***) I have cleaned the IAC, and other sensors before but not the TB so I will see. Hopefully I can Fix it. The RPMs only go up and down about 100 RPM which my mechanic said is normal. But I like when it's steady...